With the rapid ongoing development of the information society, there is considerable demand for further increases in the capacities of hard disk devices (HDD). In order to increase the surface recording density of a magnetic recording medium used in an HDD, the grain size of the magnetic crystal grains must be reduced. However, a problem arises in that as the magnetic grains are reduced in size, the thermal stability of the grains tends to deteriorate.
The expression KuV/kT (wherein Ku represents the crystal magnetic anisotropy constant, V represents the magnetic grain volume, k represents Boltzmann's constant, and T represents the absolute temperature) is generally used as an indicator of the thermal stability, and the value of KuV/kT must typically be 60 or more to ensure satisfactory thermal stability. Because the value of V decreases as the magnetic grains are reduced in size, the value of KuV/kT also decreases, resulting in a deterioration in the thermal stability.
In order to prevent this deterioration in thermal stability, the value of Ku must be increased, but increasing Ku causes an increase in the anisotropic magnetic field Hk. This is due to the relationship represented by Ku=(Ms×Hk)/2. If the value of Hk exceeds the recording magnetic field of the recording head, then satisfactory writing becomes impossible, and therefore Hk must be set to a value that is lower than the recording magnetic field.
This requirement determines the upper limit for Ku, and therefore the lower limit for miniaturization of the crystal grains. In order to realize a surface recording density at the 1 Tbit/inch2 level, the magnetic grain size must be reduced to approximately 5 to 6 nm. However, considering that the recording magnetic field Hw of conventional recording heads is typically 10 to 12 kOe and the requirement for a value of KuV/kT is greater than 60, the limit for miniaturization of the magnetic grains is approximately 10 nm.
One technique that has been proposed to overcome this problem is heat-assisted recording. Heat-assisted recording is a recording technique in which near-field light or the like is irradiated onto the medium, thereby causing localized heating of the medium surface and reducing the coercive force of the medium, thus enabling writing to be performed. In this case, even with a recording medium having a coercive force at room temperature of several tens of kOe, the recording magnetic field of a conventional magnetic head is readily capable of writing to the medium. Accordingly, by using a material having a high Ku value in the order of 106 J/m3 for the recording layer, the magnetic grain size can be reduced to 6 nm or less, while retaining favorable thermal stability. Known examples of this type of high Ku material include FePt alloys (Ku: approximately 7×106 J/m3) and CoPt alloys (Ku: approximately 5×106 J/m3) having an L10 crystal structure.
In heat-assisted recording, the magnetic layer preferably adopts a granular structure in which high Ku magnetic crystal grains of an aforementioned FePt alloy or the like are divided into segments by an oxide such as SiO2. By employing a granular structure, exchange interaction between the magnetic grains is reduced, and the grain size of the magnetic crystal grains can also be reduced.
Patent Documents 1 to 3 disclose techniques related to heat-assisted recording. Patent Document 1 relates to a magnetic recording medium in which, following formation of a recording layer composed of FePt nanoparticles, the recording layer is irradiated with a laser beam spot to promote crystallization and orientation of the recording layer. Further, Patent Document 2 relates to a magnetic recording medium that includes an FePt alloy having an L10 crystal structure. Moreover, Patent Document 3 discloses a patterned thin film provided with regions of low thermal conductivity within a material of high thermal conductivity, and also discloses a structure in which this type of thin film is used as a temperature control layer within a heat-assisted magnetic recording medium.
Non-Patent Document 1 discloses that by adding 38% of SiO2 to FePt, the magnetic grain size can be reduced to 5 nm. Further, the same document also discloses that by further increasing the amount of added SiO2 to 50%, the grain size can be reduced to 2.9 nm.